and save as /usr/share/gnome-shell/search_providers/duckduckgo.xml for instance on Debian Wheezy. Now, refresh Gnome Shell (by doing alt+f2, ‘r’) and whenever you search for someting in the Shell’s Activity overview, you have the possibility to search the Web with DuckDuckGo.

Yes, you are right, DuckDuckGo uses several other crawlers (bing, blekko, yahoo, etc.)
However it does not impact your own personal searches, and most of duckduckgo’s sources are published as Free Software.